In an image forming apparatus such as an electro-photographic copying apparatus and a printer, in recent years, a belt type fixing apparatus has been developed. The belt type fixing apparatus can set a long fixing time, so that even in a color image composed of superimposed toners, a satisfactory fixing property can be obtained. Some belt type fixing apparatus stretches an endless fixing belt between a heating roller and a separation roller, heats the fixing belt by the heating roller, and inserts a sheet of paper through a nipper section between the fixing belt and a pressure roller to heat, pressurize, and fix a toner image. The fixing belt, to obtain high-speed and satisfactory fixing, is required to shorten the warming-up time and retain a predetermined fixing temperature. For the purpose, the separation roller is required to use a highly heat insulation roller to prevent the fixing belt from cooling when it makes contact with the fixing belt and furthermore is required to reduce the weight to realize compactness, lightweight, and energy conservation of the apparatus.
Conventionally, in Japanese Patent Application Publication No. 2004-109649, an apparatus having a plurality of heat pipes installed inside a central part of a core bar of a fixing roller is disclosed.
However, the pipes are structured so as to be installed inside the core bar and uniformly diffuse heat to the core bar. Namely, the pipes do not form a heat insulation structure between the hollows of the pipes and the core bar. Further, as a heat insulating separation roller, there are a roller composed of heat resistant silicone rubber or heat resistant sponge and a ceramic roller covered with a PFA (perfluoroalkyl vinyl ether) tube available. However, the heat resistant silicone rubber and heat resistant sponge have comparatively high thermal conductivity, thus a higher heat insulating effect cannot be obtained, and the warming-up time cannot be shortened. Furthermore, the ceramics roller produces a heat insulation effect by an air layer contained in ceramics, contributes to shortening of the warming-up time, though is difficult to ensure the manufacturing accuracy, requires a damage prevention measure, thereby is comparatively expensive.
Therefore, in the separation roller for driving the fixing belt, to prevent the fixing belt from loss of temperature, it is desired to use a heat insulation roller which is low in heat capacity, high in heat insulation, and light in weight, has a satisfactory fixing efficiency, and can obtain a fixed image of high image quality.